1. Field of the Invention
The present invention relates to a method of manufacturing an electro-optic device represented by a liquid crystal display, to a method of manufacturing a semiconductor device available to the construction of the liquid crystal display, and to a method of selectively forming a thin-film device which is different in its characteristics.
2. Description of the Related Art
In recent years, there have been known liquid-crystal displays using a thin-film transistor. This is called an active matrix type in which thin-film transistors are disposed in a plurality of pixels that are arranged in the form of a matrix, respectively, so as to control the storage and discharge of an electric charge held in a pixel electrode of each pixel. The active matrix type liquid-crystal display has been expected to be used as a main one in the display field in the future because it is capable of displaying a fine and high-speed moving picture.
Also, there have been known the active matrix type liquid-crystal display of a construction in which a thin-film transistor disposed in a pixel region and a thin-film transistor constituting a peripheral driver circuit for driving the pixels are formed on the same substrate. With the application of such a construction, the pixel region and the peripheral driver circuit are capable of being integrated as a thin-film integrated circuit on one substrate, to thereby remarkably promote the weight-lighting and thinning of the liquid crystal display.
In general, the required characteristics are different between the thin-film transistor disposed in the pixel region and the thin-film transistor disposed in the peripheral driver circuit. The thin-film transistor disposed in the pixel region is required to have a small off-current (also called "leak current"). The "off-current" of the thin-film transistor is directed to a current which is allowed to flow between a source and drain of the thin-film transistor in a state where the thin-film transistor is off. The "large off-current" causes the electric charge to flow out as the off-current without being held in the pixel electrode regardless of the thin-film transistor being off, thereby coming into a state in which an image display cannot be conducted in a required period of time.
On the other hand, the thin-film transistor disposed in the peripheral driver circuit is required to have a characteristic that enables a large on-current to flow, and also enables a high-speed operation to be conducted. In other words, it is required to provide a high mobility. The thin-film transistor disposed in the pixel region is not required to provide the high mobility because it is not required to conduct a high-speed operation so much.
In this way, the thin-film transistor disposed in the pixel region and the thin-film transistor disposed in the peripheral driver circuit are required to provide a different characteristic, respectively. Therefore, in the case of integrating on the same substrate the thin-film transistor disposed in the pixel region and the thin-film transistor disposed in the region of the peripheral driver circuit, a thin-film transistor having a required characteristic need be selectively formed in a required region through a devised manufacture process.